To create multi-colored or white light based on additive color mixing principles, often multiple different sources of colored light are employed, for example red light, blue light and green light, corresponding to the “primary” colors of human vision. These three primary colors roughly represent the respective spectral sensitivities typical of the three different types of cone receptors in the human eye (having peak sensitivities at wavelengths of approximately 650 nanometers for red, 530 nanometers for green, and 425 nanometers for blue) under photopic (i.e., daytime, or relatively bright) viewing conditions. Much research has shown that additive mixtures of primary colors in different proportions can create a wide range of colors discernible to humans.
Accordingly, based on additive mixing principles, a lighting device (hereinafter referred to as a lighting fixture or lighting unit) may be configured to generate variable color light or variable color temperature white light by employing multiple different source spectrums. In particular, a resulting spectrum of perceived light provided by the lighting unit is determined primarily by the relative amounts of radiant output power associated with the respective different source spectrums that are added together (for purposes of the present disclosure, each different source spectrum of such a lighting unit also may be referred to as a “channel,” and the lighting unit may be referred to as a “multi-channel” lighting unit).
For example, consider a multi-channel lighting unit comprising a red channel, a green channel, and a blue channel (an R-G-B lighting unit), wherein each of a red channel contribution, a green channel contribution, and a blue channel contribution to the resulting spectrum may be specified (e.g., by some instruction or “lighting command”) in terms of a percentage of the total available operating power for the channel (i.e., 0-100% for each channel). The total available operating power for a given channel may in turn be determined, for example, by the maximum voltage applied to, and the maximum average current drawn by, one or more light sources configured to generate the particular spectrum associated with the channel.
Hence, a lighting command of the format [R, G, B]=[100%, 100%, 100%] would cause the exemplary R-G-B lighting unit to generate maximum radiant output power for each of red, green and blue channels, thereby creating white light (as well as generating a maximum thermal power associated with operation of the light sources). More generally, a command calling for 100% of available operating power for each channel would correspond to a maximum total power consumption by the lighting unit, some of which is converted to radiant output power and some of which is converted to thermal power dissipated by the lighting unit. A command of the format [R, G, B]32 [50%, 50%, 50%] also would generate light perceived as white, but less bright than the light generated in response to the former command (and with less thermal power generation, and less overall power consumption). A command of the format [R, G, B]=[100%, 0, 100%] would cause the lighting unit to generate maximum radiant output power for each of the red and blue channels, but no green output, thereby creating relatively bright purple light. Accordingly, it may be appreciated that a lighting command representing a prescribed percentage of available operating power for each channel of a multi-channel lighting unit essentially determines both the perceived color and brightness of the light generated by the lighting unit, as well as the thermal power generated by the lighting unit.
In various implementations, each different source spectrum in such a lighting unit may be generated by one light source or multiple light sources configured to generate substantially the same spectrum of light; in this manner, a lighting unit may include multiple light sources arranged in groups according to spectrum, wherein same-spectrum light sources are energized together (i.e., controlled as a group) in response to lighting commands. Additionally, the different-spectrum sources of a lighting unit may be configured to generate relatively narrow-band spectrums of radiation (e.g., essentially monochromatic sources corresponding approximately to the primary R-G-B colors of human vision), or relatively broad-band spectrums of radiation; hence, such lighting units may include narrow-band sources, broad-band sources, or a combination of various bandwidth and peak wavelength sources.
To determine a maximum operating power for each channel of a multi-channel lighting unit, an overall power handling capability of the lighting unit often is considered. In general, a maximum power handling capability of a lighting unit relates primarily to a heat dissipation capability of the lighting unit, or a maximum thermal power capacity which is not to be exceeded during operation (typically determined by an overall structure or housing configuration for the lighting unit). The maximum power handling capability of a given lighting unit typically is expressed in terms of a maximum total operating power (i.e., power consumption) in Watts (again, some of which represents the radiant output power of the generated light, and some of which represents thermal power associated with operation of the light sources). In designing multi-channel lighting units, it is often customary to divide the maximum power handling capability of the lighting unit by the number of channels in the lighting unit to arrive at a maximum power per channel. In this manner, if a desired light output requires a maximum contribution (i.e., 100%) from each of the different channels, damage to the lighting unit due to excessive thermal power generation may be avoided.
To illustrate this concept, consider a relatively straightforward example in which a maximum power handling capability of a lighting unit is given as 100 Watts, and that the lighting unit includes two different source spectrums or channels. In this example, the maximum operating power for each channel conventionally would be specified as 50 Watts (i.e., 100 Watts divided by two channels). Accordingly, if a lighting command has the format [C1, C2], wherein C1 and C2 represent the respective prescribed first and second channel percent operating powers, the lighting command [C1, C2]=[100%, 100%] would correspond to an operating power of 50 Watts for each of the first and second channels. Table 1 further illustrates this concept below for a number of different lighting commands [C1, C2] based on this example:
TABLE 1C1C2TotalC1C2OperatingOperatingOperatingcommandcommandPowerPowerPower100% 0%50 W 0 W50 W100% 50%50 W25 W75 W100%100%50 W50 W100 W  50%100%25 W50 W75 W 0%100% 0 W50 W50 W 50% 50%25 W25 W50 W 25% 25%12.5 W  12.5 W  25 W
A generalized formula for a prescribed operating power Px of a given channel in response to an arbitrary channel command Cx from 0 to 100%, based on the power allocation methodology represented by the example of Table 1 above, may be given as
                                          P            x                    =                                    C              x                        ⁡                          (                                                P                  max                                N                            )                                      ,                            (        1        )            where Pmax denotes the maximum power handling capability of the lighting unit, and N is the number of different channels in the lighting unit. As mentioned above, the prescribed operating power Px of a given channel in turn dictates the voltage applied to, and the average current permitted to be drawn by, one or more light sources configured to generate the particular spectrum corresponding to the channel. Hence, in response to an arbitrary channel command Cx, a particular voltage and current is applied to the light source of the channel such that the prescribed operating power Px is consumed, and a corresponding radiant output power of light is generated for the channel.